doc: add pidfile

issue #67

doc/knot.texi

@@ -172,6 +172,7 @@ Statement Definition and Usage
 * nsid::
 * storage::
 * rundir::
+* pidfile::
 * workers::
 * user::
 * max-conn-idle::

doc/reference.texi

@@ -40,6 +40,7 @@ else.
   [ @code{nsid} ( @code{"}@kbd{string}@code{"} | @kbd{hex_string} )@code{;} ]
   [ @code{storage} @code{"}@kbd{string}@code{";} ]
   [ @code{rundir} @code{"}@kbd{string}@code{";} ]
+  [ @code{pidfile} @code{"}@kbd{string}@code{";} ]
   [ @code{workers} @kbd{integer}@code{;} ]
   [ @code{user} @kbd{string}[@code{.}@kbd{string}]@code{;} ]
   [ @code{max-conn-idle} ( @kbd{integer} | @kbd{integer}(@code{s} | @code{m} | @code{h} | @code{d})@code{;} ) ]
@@ -62,6 +63,7 @@ else.
 * nsid::
 * storage::
 * rundir::
+* pidfile::
 * workers::
 * user::
 * max-conn-idle::
@@ -162,6 +164,20 @@ system @{
 @}
 @end example
 
+@node pidfile
+@subsubsection pidfile
+@vindex pidfile
+
+Specifies a custom PID file location.
+
+Default value: @file{knot.pid} in @code{rundir} directory.
+
+@example
+system @{
+  pidfile "/var/run/knot/knot_dmz.pid";
+@}
+@end example
+
 @node workers
 @subsubsection workers
 @vindex workers